Conventionally, there has been a technology for collecting measurement data by deploying a large number of nodes with a sensor across a large area of the ground, such as a plain, a slope, or a field, or in a large building, for example, and by connecting the nodes over a multi-hopping network. Such a multi-hopping network enables nodes that are remotely located to communicate with one another by allowing a node to communicate with a nearby node via a short distance wireless communication, and to transfer packets one after another. In the explanation hereunder, a network for collecting measurement data from such nodes over a multi-hopping network is referred to as a wireless sensor network (WSN).
To measure the position of a node in a WSN, there has been a conventional technology 1 for providing a node with a global positioning system (GPS). There has been another conventional technology 2 in which the position of a node is measured by conducting a land survey and recording the position at the time of installation of the node, provided that the node will be kept unmoved. There has been another conventional technology 3 for identifying the position of a node at an unknown position based on the principle of triangulation, by measuring the distance from each of a plurality of nodes at known positions to the node based on a radio field strength.
The conventional technology 1 has a disadvantage that the cost of a node is increased, and the conventional technology 2 has a disadvantage that the installation cost is increased. Especially when the number of nodes is increased, the costs of the nodes and the installation costs are increased proportionally.
With the conventional technology 3, the node of which position can be identified is limited to a node at a short distance with respect to the node at the known position, because this technology uses a short distance wireless communication. It is still possible to measure the positions of nodes in a wide area by repeating this process based on the node at the position having been identified. However, as a node is located further, the error accumulates more. Therefore, it is difficult to identify the position of a large number of nodes accurately.
As an exemplary technology addressing the disadvantages in the conventional technologies 1 to 3, a conventional technology 4 has been available. The conventional technology 4 measures the position of a node by issuing an instruction for emitting light to the node, and detecting a light-emitting point with a camera. With the conventional technology 4, the cost of a node is increased because a light-emitting component is installed in the node, but the cost can be suppressed by using a light emitting diode (LED) which is a general-purpose component, as the light-emitting component. As another technology using an LED, there has been a conventional technology 5 that notifies the identification information on a light-emitting node via an optical communication, using on and off of the light. A related art examples are disclosed in International Publication Pamphlet No. WO 2012/023253, Japanese Laid-open Patent Publication No. 2000-55657, and Japanese Laid-open Patent Publication No. 2007-170848.
However, the conventional technology has a disadvantage about measuring the position of a node using a multi-hopping communication.
The conventional technology 4 is characterized in that the time at which and the order in which packets arrive at a destination node cannot be guaranteed because the packets are sent to a destination node by causing intermediary nodes to transfer the packets to the destination node. Let us assume herein an example in which a light-emitting instruction is transmitted to a node A, but no emission of light is acknowledged after some waiting time, so that another light-emitting instruction is transmitted to a next node B. In such a case, it is impossible to measure the positions of the nodes A and B even if an emission of light is acknowledged, because it is unknown which one of the node A and the node B has emitted the light first.
The conventional technology 5 enables a mapping relation between a light-emitting point and a node to be maintained, even when the order at which the nodes emit light changes, by transmitting the identification information on a node with an on-and-off pattern of light. However, because an LED is a light source with a low luminance and no directivity, the identification information on a node can only be received at a short distance. In order to observe the light emissions across a much wider area that is assumed with a WSN at once, the camera needs to be placed distantly to observe the light emissions. Therefore, it will be difficult to use an LED in the transmission of identification information on the node, thereby making it impossible to measure the position of a node. When the use of an LED is assumed, a lower bit rate may also be used, but a lower bit rate will disadvantageously require a longer time to complete a transmission of information.
According to one aspect, an object of the present invention is to provide a position measuring apparatus, a position measuring method, and a position measuring program capable of measuring the position of a node.
According to a first aspect, a position measuring apparatus includes a control unit. The control unit instructs a first node that is included in a plurality of nodes to emit light by transmitting a light emission pattern to the first node. When light emission following the instructed light emission pattern is detected, the control unit stores information on a position at which the light emission is detected and information on the first node in a storage unit in a manner mapped to each other. When the light emission following the instructed light emission pattern is not detected, the control unit instructs a second node that is different from the first node to emit light by transmitting a light emission pattern that is different from the light emission pattern instructed to the first node, to the second node.